1. Field of the Invention
The present application relates generally to packages for LEDs, and relates more particularly to an LED package that can be manufactured completely automatically.
2. Description of Related Art
Light emitting diodes (LEDs) are typically wired through a connection to a circuit board that can be prefabricated to accept a standard LED, or custom made to receive LED connections to permit connectivity to an operating circuit and other circuit components. One type of standard LED has elongated pins that are accepted through holes in a printed circuit board (PCB), and then soldered in place to provide a mechanical and electrical bond to the PCB. The PCB typically has a number of holes and traces for mounting other components to be interconnected to form a circuit. Often, PCBs are assembled with components using automated machinery known as pick and place type robotic machines that can grasp a component, position it in an appropriate place over a desired PCB, and insert the component into the PCB. This type of circuit board manufacture is also referred to as “stuffing”. Once a printed circuit board has been assembled to a state in which the components can be soldered, the printed circuit board is often passed through a soldering device that operates at a typically high temperature with regard to component tolerances, to reflow solder that has been previously deposited on the PCB, in addition to obtaining a good mechanical and electrical bond between added solder and board components.
Another technique for manufacturing circuit boards with components involves surface mount technology (SMT), in which a component is mounted to a PCB surface, rather than into PCB holes. SMT components are often much smaller in size than equivalent thru-hole mounted devices, and therefore obtain the advantage of further reduction in PCB size. Similar to thru-hole mounted devices, SMT devices are often picked and placed by automatic equipment that is programmed to precisely position an SMT device over a selected portion of a PCB. The PCB is provided with solder pads that align with connections provided on the SMT device to be mounted, and the positioning equipment places the SMT device precisely over the solder pads during the manufacturing process. Once the SMT device is placed in contact with the solder pads, an infra-red (IR) convection or similar process heats the PCB and circuitry components to a reflow temperature of the solder paste, which is often a lead tin combination. Accordingly, the heat profile of the reflowed solder has an average peak temperature of approximately 220° C.
It is often the case that some SMT devices are susceptible to the temperatures involved in a reflow process, in that they can suffer degradation or damage if they are heated above a certain threshold temperature for a certain period of time. An SMT device that is sensitive to heat levels that may be required in the reflow process are often packaged with a protective heat transfer cover or other substrate, for example. In the case of SMT LED devices, an LED die is coupled to a printed circuit substrate having solder pads for attachment to a PCB. The LED die is attached to the upper surface of the printed circuit substrate with a protective transparent transfer molded cover. The protective cover is provided for the purpose of protecting the LED die against experiencing damaging temperatures during a solder reflow process. The protective cover does not provide any kind of lensing or optical properties for use with the LED.
LED light output is often used for signaling purposes, such as in the case of indicators, and it is therefore desirable to direct the LED light output in a particular direction or with particular characteristics to accomplish a design goal. If no lensing or optical control is provided, the LED light output is somewhat inefficient in accomplishing the purpose for which it is often intended.
During the manufacture of devices having SMT LEDs, a lensed or optically controlled light package is obtained by placing a separate, secondary, optical component over the LED package to modify the light output of the LED. Often, the secondary optical component is a waveguide or optical lens that directs light in an intended direction according to a particular application.
Although it would be desirable to provide a lens or optical component to the LED at the time of manufacture with the PCB, the desired materials for providing suitably precise lensing or optical control are unable to withstand the reflow soldering process. The lensing or optical component is therefore coupled to the LED after the reflow process when the PCB has cooled sufficiently. Typically, SMT components do not require any further processing after the reflow process, since the advantages of SMT processing lies in automated manufacture and smaller sizes without human intervention. Accordingly, it becomes very inefficient and defeats the purpose of surface mount technology if an additional step is required after the reflow process with human intervention. However, due to the nature of SMT LED devices, and the unavailability of material suitable for lensing or optical control that are desirable for use with SMT LEDs, an additional placement process must be provided after the reflow process to achieve a finished PCB. That is, the lensing or optical control component must be placed by hand, typically, on the SMT LED device after the reflow process is completed. For example, one conventional method consists of a lens coupled to an opaque box, which is then glued on top of a typical or standard SMT LED. The process is time consuming and costly, while diminishing the value obtained in the SMT processing.